Abstract: "The isolated skulls of _Nemegtosaurus mongoliensis_ and
_Quaesitosaurus orientalis from the Nemegt Basin of Mongolia are among the
most complete sauropod cranial remains known from the Late Cretaceous, yet
their evolutionary relationships to other neosauropods have remained
uncertain. Redescription of the skull of _Nemegtosaurus_ identifies key
features that link it and its closely related counterpart _Quaesitosaurus_
to titanosaur sauropods. These include a posterolaterally orientated
quadrate fossa, `rocker'-like palatobasal contact, pterygoid with reduced
quadrate flange and a novel basisphenoid?quadrate contact. Other features
are exclusive to
_Nemegtosaurus_ and _Quaesitosaurus_, such as the presence of a symphyseal
eminence on the external aspect of the premaxillae, a highly vascularised
tooth bearing portion of the maxilla, an enclosed `maxillary canal', orbital
ornamentation on the postorbital, prefrontal and frontal, exclusion of the
squamosal from the supratemporal fenestra and dentary teeth smaller in
diameter than premaxillary and maxillary teeth.
"Re-examination of Late Cretaceous sauropod distributions in the light of
this well-supported phylogenetic hypothesis has important implications for
their diversity at the end of the Mesozoic in Asia and elsewhere.
Cretaceous Asian sauropod faunas consist solely of titanosauriforms, which
probably migrated there from other landmasses during the Late Jurassic,
during which time neosauropods were absent from Asia. Globally,
narrow-crowned titanosaurs and rebbachisaurids radiated during the
Cretaceous, but only titanosaurs survived into the latest Cretaceous. These
late-surviving sauropods flourished on most continental landmasses until the
end of the Maastrichtian."

Wilson (2005) re-defines the Nemegtosauridae as the stem-based clade
including all titanosaurs more closely related to _Nemegtosaurus_ than to
_Saltasaurus_. This group includes (at least): _Nemegtosaurus_,
_Quaesitosaurus_, _Rapetosaurus_, possibly _Mongolosaurus_ (which is
regarded as a valid genus) and maybe even _Phuwiangosaurus_ (although the
postcranium indicates that it was a more basal titanosaur).
_Quaesitosaurus_ is not considered a junior synonym of _Nemegtosaurus_,
though the two are sister taxa. _Antarctosaurus wichmannianus_ may also be
a nemegtosaurid; but Wilson is not certain that all the material referred by
Huene to _A. wichmannianus_ comes from a single individual or species.

Like many previous authors, Wilson also raises the possibility that
_Nemegtosaurus_ and _Opisthocoelicaudia_ may be the same; but overlapping
material is required before this can be considered. Considering that
Nemegtosauridae and Opisthocoelicaudiinae have both received phylogenetic
definitions, such a synonymy would raise interesting nomenclatural issues...

Wilson (2004) also notes that titanosaurs and rebbachisaurids appear to be
the only two sauropod clades that flourished in the Cretaceous. Although
other groups (brachiosaurids, dicraeosaurids, basal diplodocoids) survived
into the Cretaceous "they did not appear to diversify".